The shape and location of groups will change with powder charges. Notice the first to rows and the charge changes from left to right for both the 155 SMK and the 155.5 Berger.
This is repeatable on different days as the picture below shows when I test RWS brass with the 155.5 Berger.
On the left is the same bullet and powder at three different charges of powder. Notice that the shape of the group changes
One thing that will goof you up big time is mirage. Mirage with a twitchy gentle breeze will drive you crazy. Shoot on cloudy days if possible.